This invention relates to apparatus capable of continuously measuring the formation of paper while it is being produced by a papermaking machine.
The term "formation", as applied to paper, means the relative uniformity of distribution of fibres in the paper sheet. Nonuniformity, or poor formation, commonly arises because flocculation occurs in the fibre suspension from which the paper is made. This flocculation may occur prior to the delivery of the suspension to the drainage section of the paper machine or in the drainage section itself, and in either case results in poor formation. Formation depends on complex interactions of equipment, fiber and paper making parameters. Formation nonuniformity occurs as variations in mass per unit area over a relatively narrow range of distances commonly accepted to be within 0.1 to 10 cm. The term "floc" is used to denote a small area of mass per unit area above the average for the web. The characterization of formation therefore requires data concerning the relative size and spatial distribution of the flocs in a sheet of paper as well as the nonuniformities in weight distribution resulting from equipment related effects such as nonuniform distribution of flow to the fluid removal section of the machines.
Formation is an important parameter of paper quality since it affects many end use performance characteristics. For example variations of mass distribution of the dimensions of formation (i.e. 0.1-10 cm) interact with the calendering process to produce variations of density of the same dimensions. This can seriously affect printing quality in certain processes such as gravure printing where a high print gloss is desirable. Good formation produces uniform sheet density and uniform print gloss while poor formation produces nonuniform sheet density and an undesirable print mottle.
The traditional technique of judging formation has been by visual examination of samples in transmitted light by an experienced observer. This is capable of general comparisons of a limited number of samples but does not produce numerical results. It is also vulnerable to differences between observers and with time. Instruments that measure formation and produce numerical results are in existence. Most, however, are off machine units that measure a sample of paper remote from the paper machine. While this is a valuable method it does have limitations. Formation is a dynamic characteristic that can vary with subtle changes in the forming section. Measurements with an off machine instrument are limited to the end of the reel or require the loss of considerable paper to get to samples within a reel. These limitations are eliminated if the formation measurement is carried out on the machine in real time. In that way cause and effect can be closely associated.
The basic principal of most if not all formation instruments is the measurement of the attenuation of a beam of light as it is transmitted through the web. This beam scans the sheet and produces a signal proportional to the weight of the sheet that is characteristic of the formation. The differences between instruments lie in the technique of scanning the sheet and in obtaining a usable index of formation from the very complex signal obtained from the sensor. In off machine instruments some mechanical scanning device is provided. For on machine instruments the motion of the paper through the machine provides part of the scanning facility. The sensor may remain stationary on the machine and produce a reading alone one line in the machine direction of the paper or it may be mounted on a scanning device which reciprocates across the width of the web. This produces a measurement that covers both the length and width of the paper being processed.
U.S. Pat. Nos. 3,114,791, which issued Dec. 17, 1963 to Zabel, et al. and 3,525,871, which issued Aug. 25, 1970 to Lehtinen, represent examples of on line formation analysers. The Zabel patent uses two phototubes which serve as sensing elements of a scanning device, these being spaced apart to sense different areas. The net signal derived from the phototubes is a differential signal representing the difference in light intensity on the phototubes at any given instant in time. The Lehtinen patent describes a feedback system which compensates for overall changes in the light transmittance of the paper.